Life on a Changing Planet

How have our planet's environment and living things affected each other to shape the history of life on Earth?

Today, it's easy to think of places on Earth where the environment is relatively constant from year to year. Arizona is dry, coastal Washington State is wet, Antarctica is cold, and the Sahara is hot. But this was not always the case. Earth's physical environment has undergone striking changes in its history, and many of these changes have affected life in dramatic ways.

FIGURE 19–7 The Changing Face of Earth Over the last 225 million years, the face of the Earth has changed dramatically.

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Physical Forces Climate is one of the most important aspects of the physical environment, and Earth's climate has been anything but constant over the history of life. Many of these changes were triggered by fairly small shifts in global temperature. For example, during the global “heat wave” of the Mesozoic era, average global temperatures were only 6°C to 12°C higher than they were in the twentieth century. During the great ice ages, which swept across the globe as recently as 10,000 years ago, temperatures were only about 5°C cooler than they are now. Yet, these temperature shifts had far-reaching effects on living things.

Geological forces have also transformed life on Earth, building mountains and even moving whole continents. Remember that local climates are influenced by the interactions of wind and ocean currents with geological features like mountains and plains. Volcanic forces have altered landscapes over much of Earth, even producing entire islands that provide new habitats. The Hawaiian Islands, home to scores of unique plant and animal species, are a perfect example of how volcanic islands can alter the course of evolution. Building mountains, opening coastlines, changing climates, and geological forces have altered habitats of living organisms repeatedly throughout Earth history.

Over the long term, the process of continental drift has produced even more dramatic changes in Earth's biological landscape. As shown in Figure 19–7, continents have collided to form “super continents,” and then drifted apart again, profoundly changing the flow of ocean currents. Continental drift has also affected the distribution of fossils and living organisms worldwide. For example, the continents of Africa and South America are now separated by the Atlantic Ocean. But fossils of Mesosaurus, an aquatic reptile, have been found in Africa and South America. The presence of these fossils on both continents reflects the fact that both were joined at one time. The theory of plate tectonics explains these movements as the result of solid “plates” moving slowly, as little as 3 cm a year, over Earth's mantle.

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